I. Field of the Disclosure
The technology of the disclosure relates generally to magnetic tunnel junction (MTJ) devices, which may be employed in a resistive memory, such as a magnetic random access memory (MRAM) for example, and more particularly to fabrication of MTJ devices.
II. Background
Semiconductor storage devices are used in integrated circuits (ICs) in electronic devices to provide data storage. One example of a semiconductor storage device is magnetic random access memory (MRAM). MRAM is non-volatile memory in which data is stored by programming a magnetic tunnel junction (MTJ) as part of an MRAM bit cell. One advantage of an MRAM is that MTJs in MRAM bit cells can retain stored information even when power is turned off. This is because data is stored in the MTJ as a small magnetic element rather than as an electric charge or current.
In this regard, an MTJ comprises a free ferromagnetic layer (“free layer”) disposed above or below a fixed or pinned ferromagnetic layer (“pinned layer”). The free and pinned layers are separated by a tunnel junction or barrier formed by a thin non-magnetic dielectric layer. The magnetic orientation of the free layer can be changed, but the magnetic orientation of the pinned layer remains fixed or “pinned.” Data can be stored in the MTJ according to the magnetic orientation between the free and pinned layers. When the magnetic orientations of the free and pinned layers are anti-parallel (AP) to each other, a first memory state exists (e.g., a logical ‘1’). When the magnetic orientations of the free and pinned layers are parallel (P) to each other, a second memory state exists (e.g., a logical ‘0’). The magnetic orientations of the free and pinned layers can be sensed to read data stored in the MTJ by sensing a resistance when current flows through the MTJ. Data can also be written to and stored in the MTJ by applying a magnetic field to change the orientation of the free layer to either a P or AP magnetic orientation with respect to the pinned layer.
Recent developments in MTJ devices involve spin-transfer torque (STT)-MRAM devices. In STT-MRAM devices, the spin polarization of carrier electrons, rather than a pulse of a magnetic field, is used to program the state stored in the MTJ (i.e., a ‘0’ or a ‘1’). FIG. 1 below illustrates an MTJ 100. The MTJ 100 is provided as part of an MRAM bit cell 102 to store non-volatile data. A metal-oxide semiconductor (MOS) (typically N-type MOS, i.e., NMOS) access transistor 104 is provided to control reading and writing to the MTJ 100. A drain (D) of the access transistor 104 is coupled to a bottom electrode 106 of the MTJ 100, which is coupled to a pinned layer 108 for example. A word line (WL) is coupled to a gate (G) of the access transistor 104. A source (S) of the access transistor 104 is coupled to a voltage source (VS) through a source line (SL). The voltage source (VS) provides a voltage (VSL) on the source line (SL). A bit line (BL) is coupled to a top electrode 110 of the MTJ 100, which is coupled to a free layer 112 for example. The pinned layer 108 and the free layer 112 are separated by a tunnel barrier 114.
With continuing reference to FIG. 1, when writing data to the MTJ 100, the gate (G) of the access transistor 104 is activated by activating the word line (WL). A voltage differential between a voltage (VBL) on the bit line (BL) and the voltage (VSL) on the source line (SL) is applied. As a result, a write current (I) is generated between the drain (D) and the source (S) of the access transistor 104. If the magnetic orientation of the MTJ 100 in FIG. 1 is to be changed from AP to P, a write current (IAP-P) flowing from the free layer 112 to the pinned layer 108 is generated. This induces an STT at the free layer 112 to change the magnetic orientation of the free layer 112 to P with respect to the pinned layer 108. If the magnetic orientation is to be changed from P to AP, a current (IP-AP) flowing from the pinned layer 108 to the free layer 112 is produced, which induces an STT at the free layer 112 to change the magnetic orientation of the free layer 112 to AP with respect to the pinned layer 108.
FIG. 2 is a schematic diagram illustrating exemplary layers of a conventional perpendicular (pMTJ) 200 provided in an MTJ stack pillar 202 that can be employed in the MTJ 100 in FIG. 1. The pMTJ 200 includes highly reliable pinned/reference layers that can be provided by high perpendicular magnetic anisotropy (PMA) materials (i.e., materials having a perpendicular magnetic easy axis). In this regard, the MTJ stack pillar 202 includes a pinned layer 204 of a high PMA material disposed on a seed layer 205 (e.g., a Tantalum (Ta)/Platinum (Pt) bilayer) above a bottom electrode 206 (e.g., made of Tantalum (Ta) Nitride (N) (TaN)) electrically coupled to the pinned layer 204. A tunnel barrier 208 provided in the form of a Magnesium Oxide (MgO) layer in this example is disposed above the pinned layer 204. The MgO tunnel barrier 208 has been shown to provide a high tunnel magnetoresistance ratio (TMR). A free layer 210, shown as a Cobalt (Co)-Iron (Fe)-Boron (B) (CoFeB) layer in this example, is disposed above the tunnel barrier 208. The CoFeB free layer 210 is a high PMA material that allows for effective current-induced magnetization switching for a low current density. A conductive, non-magnetic capping layer 212, such as a thin Magnesium Oxide (MgO) and/or Tantalum (Ta) material for example, is disposed above the free layer 210 to protect the layers of the MTJ stack pillar 202. A top electrode 214 is disposed above the capping layer 212 to provide an electrical coupling to the free layer 210.
In the MTJ stack pillar 202 in FIG. 2, the magnetic orientation of the pinned layer 204 is fixed. Accordingly, the pinned layer 204 generates a constant magnetic field, also known as a “net stray dipolar field,” that may affect, or “bias,” a magnetic orientation of the free layer 210. This magnetic field bias, at best, can cause an asymmetry in the magnitude of current necessary to change the magnetic orientation of the free layer 210 (i.e., IP-AP is different than IAP-P). The current necessary to change the magnetic orientation of the free layer 210 towards the bias orientation is reduced while the current necessary to change the magnetic orientation of the free layer 210 against the bias is increased. At worst, this magnetic field bias can be strong enough to “flip” the value of a memory bit cell employing the pMTJ 200 in FIG. 2, thus decreasing the reliability of the subject MRAM. To reduce or prevent a magnetic field bias being provided by the pinned layer 204 on the free layer 210, the pinned layer 204 in the MTJ stack pillar 202 in FIG. 2 includes a synthetic anti-ferromagnetic (SAF) structure 216. The SAF structure 216 includes a hard, first anti-parallel ferromagnetic (AP1) layer and a second anti-parallel ferromagnetic (AP2) layer separated by a non-magnetic anti-ferromagnetic coupling (AFC) layer 218 (e.g., a Ruthenium (Ru) layer). The AP1 and AP2 layers are permanently magnetized and magnetically coupled in opposite orientations to generate opposing magnetic fields. The opposing magnetic fields produce a zero or near-zero net magnetic field towards the free layer 210, thus reducing the magnetic field bias problem at the free layer 210.
MTJ patterning or etching processes are used to fabricate MTJs, such as the MTJ stack pillar 202 in FIG. 2. MTJ etching involves the need to etch complicated metal stacks. Currently known methods for MTJ etching, especially at tight pitches, include ion beam etching (IBE) and chemical etching in a reactive ion etching (RIE), both of which have challenges. RIE processes are known to create damage zones around the perimeter of the MTJ. Etching damage in the transition metals (i.e., the pinned layer 204, the free layer 210, and the bottom and top electrodes 206, 214) in the MTJ can affect factors such as a tunnel magnetoresistance ratio (TMR) and energy barrier (Eb) variations, which can result in poor MTJ performance. Another method of MTJ etching involves IBE. IBE may be used for etching materials that have tendencies to not react well to chemical etching. An IBE process can avoid or reduce damage zones over RIE processes, but no chemical component is involved to improve etching selectivity. IBE involves directing a charged particle ion beam at a target material to etch the material.
With both RIE and IBE processes, etched metal can be redeposited at a tunnel barrier of an etched MTJ stack pillar. For example, FIG. 3 illustrates exemplary MTJ devices 300(1), 300(2) similar to the MTJ stack pillar 202 in FIG. 2 fabricated in a semiconductor wafer 302 that have metal redeposition 304(1), 304(2) around an MTJ stack pillar 306(1), 306(2) as a result of etching MTJ stacks and over-etching at the end of an MTJ device etch process. Areas in a dielectric material layer 308 adjacent to the MTJ stack pillars 306(1), 306(2) are over-etched to form over-etch trenches 310(1), 310(2) to avoid horizontal shorts between adjacent devices at smaller pitches. However, bottom electrodes 312(1), 312(2) of the MTJ devices 300(1), 300(2) are also etched as a result of this over-etching. Metal etched from the bottom electrodes 312(1), 312(2) is redeposited as the metal redeposition 304(1), 304(2) around an MTJ stack pillar 306(1), 306(2). Even tiny amounts of redeposited metal material can cause metal shorts across a tunnel barrier of the MTJ stack pillar 306(1), 306(2), because the tunnel barrier in the MTJ stack pillar 306(1), 306(2) may be as small as one (1) nanometer (nm) in height. This metal redeposition 304(1), 304(2) can lead to metal shorts. Thus, as MTJ devices become scaled down, such as in high-density MRAMs, this redeposition from over-etching can limit the amount of downscaling.